1. Field of the Invention
The present invention relates to mud-driven motors used in the drilling of wellbores for hydrocarbon production. More particularly, the invention relates to the sealing elements employed within the power section of a downhole drilling motor.
2. Background of the Related Art
The concept of downhole motors for driving an oil well drill bit is more than one hundred years old. Modem downhole motors, also known as progressive cavity motors or simply mud motors, are powered by circulating drilling fluid (mud), which also acts as a lubricant and coolant for the drill bit, through a drill string in which a downhole motor is conveyed. Prior art FIG. 1 shows a conventional downhole motor assembly. The motor assembly 10 generally includes a rotatable drill bit 12, a bearing/stabilizer section 14, a transmission section 16 which may include an adjustable bent housing, and a motor power section 18. The bent housing 16 is not an essential part of the motor assembly, and is only used in directional drilling applications. During operation, drilling fluid pumped through the drill string 20 from the drilling rig at the earth's surface passes through the motor power section 18 and exits the assembly 10 through the drill bit 12.
Prior art FIGS. 2 and 3 show details of the power section 18 of a conventional downhole motor. The power section 18 generally includes a tubular housing 22 which houses a motor stator 24 within which a motor rotor 26 is rotationally mounted. The power section 18 converts hydraulic energy into rotational energy by reverse application of the Moineau pump principle. It will be appreciated by those skilled in the art that the difference between a “motor” and a “pump” as used herein is the direction of energy flow. Thus, a progressive cavity motor may be operated as a progressive cavity pump by direct (as opposed to reverse) application of the Moineau pump principle wherein rotational energy is converted into hydraulic energy. For the sake of clarity, the term “motor” will be used hereafter to mean a device that transforms energy between hydraulic energy and rotational energy, typically (but not exclusively) in the direction of a hydraulic-to-rotational energy transformation.
The stator 24 has a plurality of helical lobes, 24a-24e, which define a corresponding number of helical cavities, 24a′-24e′. The rotor 26 has a plurality of lobes, 26a-26d, which number one fewer than the number of stator lobes and which define a corresponding plurality of helical cavities 26a′-26d′. Generally, the greater the number of lobes on the rotor and stator, the greater the torque generated by the motor power section 18. Fewer lobes will generate less torque but will permit the rotor 26 to rotate at a higher speed. The torque output by the motor is also dependent on the number of “stages” of the motor, a “stage” being one complete spiral of the stator helix.
In conventional downhole motors, the stator 24 primarily consists of an elastomeric lining that provides the lobe structure of the stator. The stator lining is typically injection-molded into the bore of the housing 22, which limits the choice of elastomeric materials that may be used. During refurbishment, the stator must be shipped to a place where the injection molding can be performed. This increases the costs of maintenance of the motors.
The rotor is typically made of a suitable steel alloy (e.g., a chrome-plated stainless steel) and is dimensioned to form a tight fit (i.e., very small gaps or positive interference) under expected operating conditions, as shown in FIG. 3. It is generally accepted that either or both the rotor and stator must be made compliant in order to form suitable hydraulic seals. The rotor 26 and stator 24 thereby form continuous seals along their matching contact points which define a number of progressive helical cavities. When drilling fluid (mud) is forced through these cavities, it causes the rotor 26 to rotate relative to the stator 24.
The following patents disclose, in varying applications, the use of elastomeric liners that are molded, extruded, or bonded (e.g., chemically, thermally) to the rotor of a downhole motor, either to supplement or to replace the elastomeric liner of the stator: U.S. Pat. No. 4,415,316; U.S. Pat. No. 5,171,138; U.S. Pat. No. 6,183,226; U.S. Pat. No. 6,461,128; and U.S. Pat. No. 6,604,922. None of these patents discloses a rotor liner that is easily replaced, presumably because the described means of molding/extruding/bonding do not facilitate easy replacement.
Accordingly, a need exists for a solution of sealing the power section of a downhole motor in such a manner that facilitates easy replacement of the sealing elements. Moreover, a need exists for such a sealing solution that does not necessitate the expensive process of relining the motor stator to maintain an adequate seal in the power section.